This invention relates to an apparatus for aerating liquids held in a containment structure. More particularly the invention relates to an apparatus for aerating liquids contained in large structures wherein the apparatus is maintained in an operating position by floatation.
In the treatment of wastewater, in the conditioning of water for aquatic life, and for various industrial and environmental processes, it is necessary to dissolve oxygen or other gases in a liquid so as to promote bacterial action, provide oxygen for survival of aquatic life, chemically oxidize substances and various other reasons. In processes requiring oxygen, it is well known to compress air, (which contains approximately 21% oxygen) and inject it in bubble form beneath the surface of a liquid so as to dissolve a portion of the oxygen of the air bubbles into the liquid being treated. Factors such as size of the bubbles, bubble residence in the liquid, temperature of the air and liquid, depth of injection, etc. determine the percentage of the oxygen that is dissolved in the liquid prior to the oxygen-containing bubbles reaching the top surface of the liquid. By optimizing various factors a more efficient aerating process can be carried out so as to maximize the oxygen dissolved per unit energy input to the aeration system. The factors contributing most to the efficiency of the system are bubble size and bubble residence in the liquid.
When bubbles are produced from a given quantity of air, the area of gas/liquid interface is greater for small bubbles formed from that quantity of air than for larger bubbles formed from that quantity of air.
Bubble residence time in a liquid is primarily dependent on 1) size of the bubble, and 2) factors other than buoyancy that move a bubble in a vertical direction toward the top surface of the liquid. Regarding vertical movement due to bubble size and buoyancy, the smaller the bubble the slower the vertical movement.
The primary factor in bubble residence, other than buoyancy and its relation to bubble size, is upwardly directed currents in the liquid which add velocity to the bubbles and decreases the time it takes a bubble to reach the top surface of the liquid. The upwardly directed currents can be caused by various conditions, however, a prime cause found with prior art aeration devices is liquid density induced currents. Liquid density induced currents are described with reference to FIGS. 1 and 2. In FIG. 1, the body of liquid 22 has portions 20, having bubbles distributed throughout, and portions 24 which are substantially free of bubbles. Such a condition is found, for example, where concentrated areas of bubble-producing devices such as 26 are spaced apart a relatively large distance (for example 20-40 ft.) in a wastewater treatment pond. When a condition as described exists, portions 20, having bubbles throughout, have a lower density than the surrounding bubble-free portions 24 and upwardly directed currents, indicated by arrows 28, are induced by density gradients.
Another example of the density induced currents is described with reference to FIG. 2. In FIG. 2, reactor tank 30, has bubble-providing devices 32 located solely along two sides of the tank. Rolling currents 34 are induced as a result of the density gradients and they increase the upward vertical velocity of bubbles 36 thus reducing the bubble residence time.
An additional problem found with some prior art aeration devices having bottom support members, experienced especially during installation or maintenance, is the need to drain the containment structure. Such a need can present enormous problems for many installations. The devices of FIGS. 1 and 2 are both bottom mounted.
The apparatus and methods of the present invention overcome those problems and other deficiencies found in prior art aerators.
The present invention includes a gas distribution network having input and output apertures for receiving a gas and conveying it to output apertures which are in communication with fine-bubble producing devices which receive the gas, form bubbles, and discharge the fine bubbles into the liquid in which the apparatus is submerged. A floatation device is used to maintain the fine-bubble producing devices at a selected depth below the top surface of the liquid absent any vertical support from structural members bearing on the containment structure holding the liquid.
The fine-bubble producing devices, such as membrane disc diffusers or membrane tube diffusers are spaced uniformly to form a two dimensional grid with a spacing which provides a substantially uniform density of bubbles above the grid. Liquid density induced currents are minimized and/or prevented from developing over a large portion of the grid because of the substantially uniform density of bubbles throughout the grid area. A grid size is determined which minimizes the affect of the unavoidable liquid density induced currents found near the periphery of the grid.
Other specific features and contributions of the invention are described in more detail with reference being made to the accompanying drawings.